Plasma processing for surface modification of trivalent chromium as alternative to hexavalent chromium layer

Plasma source ion implantation (PSII) treatment was undertaken to improve the mechanical properties of electrodeposited trivalent chromium layers. Nitrogen ions were implanted, with energies of − 15 to − 25 keV and doses of 1, 5 and 10 × 10¹⁷ atoms cm^− 2, to modify the surface properties of Cr plating layer. The surface properties of the films were characterized by XRD, SEM, ruby-ball on disk type tribometer and nanoindenter. Polycrystalline CrN films with (200), (220) and (222) orientations were preferentially grown and numbers of surface cracks were increased by N⁺-PSII onto trivalent chromium layers. The surface hardness of the Cr³⁺ plating layer was increased from 16 to 25 GPa by N⁺-PSII. Severe wear and higher friction was observed on N⁺-PSII treated trivalent Cr plating. It seemed that the wear debris from hardened and cracked surface of the N⁺-PSII treated specimen prompted abrasive wear in the wear test. Roughness of the Cr³⁺ plating layers was smoothed with increasing implantation doses.     

Thermodynamic analysis for separation of vanadium and chromium in V(IV)–Cr(III)–H2O system

To recycle vanadium and chromium from the V–Cr-bearing reducing slag, the thermodynamics of separating V(IV) and Cr(III) at 298 K was summarized in the form of potential–pH diagram and activity–pH diagram. The potential–pH diagrams of V–Mn–H₂O and Cr–Mn–H₂O systems show that the electrode potential of MnO₂/Mn²⁺ is higher than that of VO₂⁺/VO²⁺ but lower than that of Cr₂O₇^2–/Cr³⁺, which proves that it is feasible to selectively oxidize low valent vanadium using MnO₂. The activity–pH diagrams of V(V)–H₂O and Cr(III)–H₂O systems show that the precipitation pH of V(V) is far lower than that of Cr(III), and therefore V(V) and Cr(III) can be separated through precipitation method. Based on the thermodynamic analysis, the flowsheet of recovery of vanadium and chromium from the V–Cr-bearing reducing slag is designed.     

Composition analysis of the plating on electrolytically treated steel sheets in chromic acid solution

The structure and composition of the chromium plating on Tin Free Steel (TFS) were investigated and analyzed with scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDS), X-ray photoelectron spectroscopy (XPS) and coulometric electrolysis method. The plating under investigation was manufactured by a two-step electrolytic reduction from a chromic acid solution. Experimental results showed that the plating on TFS had a two-layer structure. Moreover, two valences of chromium, three and zero, were detected in the plating and no hexavalent chromium was found in it. Correspondingly, all chromium in the outer layer was trivalent and existed as the form of Cr₂O₃ and Cr(OH)₃, the content of which was about 6.62 mg/m², while the pure metallic chromium was the only form of chromium found in the inner layer with the content of about 57.75 mg/m². In addition, the amount of O existed as M-OH was twice as that of M-O in the above-mentioned hydrated chromium oxide layer.     

Preparation and characterization of Cr-P coatings by electrodeposition from trivalent chromium electrolytes using malonic acid as complex

Cr-P coatings were prepared by electrodeposition from trivalent chromium plating bath using malonic acid as complex. The influences of bath composition on the trivalent chromium electrodeposition process and deposited coating properties were studied. The effects of plating parameters such as current density, bath pH and plating time on structure and morphology of deposited coatings were investigated in detail. XRD, SEM, EDAX and XPS techniques were used to characterize the Cr-P deposited coatings. Results show that the composition, microstructure and surface morphology of the Cr-P coatings depend on bath composition and plating conditions including bath pH, current density, plating time, etc. Results of EDAX and XPS indicate that the deposited coatings contain Cr(s), Cr(III), phosphorus, oxygen and carbon. The optimum bath composition was obtained using malonic acid as complex and the mechanism of Cr-P electrodeposition was analyzed. The optimum plating parameters for good-quality chromium deposited coating are pH 2-3, current density 3-12 dm², temperature 35 °C and Ti/IrO₂ as anode. These results may be of great practical and theoretical significance for further improvement of trivalent chromium plating process.     

Corrosion effects of plasma immersion ion implantation-enhanced Cr deposition on SAE 1070 carbon steel

Chromium electroplated coatings are the most common coatings used for several decades in the general industry as a protective layer against corrosion and wear. The waste of Cr plating is an environmental problem because of the toxic and carcinogenic Cr⁶⁺ compounds resulting from the Cr baths. In this study, we suggest an alternative method for thin Cr films deposition: plasma immersion ion implantation-enhanced deposition (PIIIeD) that aims at achieving highly adherent films to carbon steel surfaces with very high resistance to corrosion attacks. Chromium films formed by PIIIeD were studied with respect to corrosion resistance, surface morphology and elements profile. Potentiodynamic scans showed a decrease in the corrosion current density, promoting higher corrosion resistance, and a hydrophobic character of the PIIIeD Cr films, turning the samples less susceptible to saline corrosion. GDOES depth profiles showed the increase of film and mixing layer thicknesses with the increase of GD power.     

Corrosion behavior of Cr electrodeposited from Cr(VI) and Cr(III)-baths using direct (DCD) and pulse electrodeposition (PED) techniques

A comparison was made between the electrochemical corrosion behaviors of chromium deposited from hexavalent [Cr(VI)] and trivalent [Cr(III)] chromium baths using direct current (DCD) and pulse electro deposited (PED) techniques. Chromium coatings were deposited on mild-steel (MS) substrate. The corrosion behavior of both DCD and PED chromium from Cr(VI) and Cr(III)-baths in 3.5%NaCl solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results indicated that PED chromium from Cr(VI) and Cr(III)-baths have higher charge-transfer resistance R_ct and very low I_corr than that of DCD chromium on mild-steel substrate.     

Electrodeposition and physico-mechanical properties of chromium coatings modified with disperse particles

Chromium electroplates from Cr(III)-baths modified with disperse phases of CeO₂, Cr₂O₃, graphite, and ultradisperse diamond are studied. Mechanical properties of the composite coatings are investigated. The Cr₂O₃-particles present in the bath are found to cause the destruction of the coatings already during the plating. When modifying with CeO₂-particles, good coatings with enhanced microhardness can be obtained at their concentration in the bath of no more than 15 g/l. The deposition rate in such cases exceeds that in the blank baths. The optimum range of the CeO₂-particle concentration, allowing to obtain the maximal microhardness, is 5–8 g/l. The self-lubricating chromium-graphite composite coatings can be deposited at a graphite concentration in the bath of up to 4 g/l. The microhardness of the chromium coatings is most strongly affected by ultradisperse diamond particles that increase the microhardness by a factor of 1.4; they also decrease the coatings’ brittleness. When the ultradisperse diamond concentration in the solution is as high as 30 g/l, the diamond content in the composite coating reaches its limiting value of 12–14 vol % (or 6.4–7.2 wt %). The optimum ultradisperse diamond concentration was found to be 17 g/l in the baths and 10.5 vol % (or 5.6 wt %) in the composite coating. At these concentrations, the coatings with the maximum microhardness and lowest brittleness are obtained. Original Russian Text ? E.G. Vinokurov, A.M. Arsenkin, K.V. Grigorovich, V.V. Bondar’, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 3, pp. 312–316.     

Characteristics of Zinc Corrosion and Formation of Conversion Films on the Zinc Surface in Acidic Solutions of Cr(III) Compounds

The influence of the components of an acidic solution: Cr(III) nitrate-malonic acid-Co(II) salt and treatment conditions on zinc dissolution and formation of Chromate films as well as on their decorative and protective properties have been studied using the analytical, XPS, structural and accelerated corrosion test methods. An organic acid is the main component, which has an essential influence on zinc dissolution and formation of Chromate films as well as their decorative and protective properties. The influence of organic acid is directly related with the state of the Cr³⁺ ions in chromating solution. When the Cr³⁺ ions are in the form of hexaaquaions, the organic acid increases the quantities of the zinc dissolved and the Cr(III) deposited on the zinc surface (especially at 60°C). It also predetermines the formation of a thick, porous Chromate film with large cracks at 60°C. Its decorative and protective properties are rather poor. When Cr³⁺ ions are in the form of a complex with organic acid, the quantities of the zinc dissolved and the Cr(III) deposited on the zinc surface significantly decrease and thinner Chromate films with an even surface, good decorative appearance and high corrosion resistance are formed Decorative blue-bright Cr(VI)-free films with a slight iridescent tint, obtained in solution, containing Cr(III) nitrate (0.2), malonic acid (0.3) and Co(II) nitrate (0.02) mol dm^?3, at p H 1.6-2.0 at room temperature over 30–60 s, possess corrosion resistance (192–240 h in a salt spray chamber) similar to that of iridescent Chromate films, obtained in acidic Cr(VI) solution.     

Thermodynamic properties and phase equilibria in the Cr-P system

A Knudsen effusion method with mass-spectrometric analysis of gaseous phase has been applied to investigate the thermodynamic properties of the chromium phosphides (1341 to 1704 K) and Cr-P liquid alloys (1664 to 1819 K). Simultaneously, DSC has been used to measure heat capacities of chromium phosphides Cr₃P and Cr₁₂P₇ in the temperature range of113 to 873 K. The entropies of formation of chromium phosphides calculated according to the second and third laws of thermodynamics agree within the limits of experimental error. The Gibbs energies of formation of the phosphides from solid Cr and P₂ gas have been approximated with the following equations (in J/mol): A_fG⁰(Cr₃P) = −(244 112 ±2800) + (70.95 ±1.80)T ΔG⁰(Cr₁₂2P₇) = −(1563 678 ±15 350) + (440.6 ±9.90)T Thermodynamic properties of liquid solutions have been described with the ideal associated-solution model assuming that CrP, Cr₂P, Cr₃P, and Cr₃P₂ complexes exist in the melt. The phase diagram computed with the help of the thermodynamic data agrees with the published information.     

Oxidation of green rust suspensions containing different chromium ion species

The oxidation of hydrosulphate green rust (GR2(SO₄²⁻)) suspension containing different chromium ion species was investigated by X-ray diffraction, X-ray absorption spectroscopy and transmission electron microscopy. The pH, oxidation-reduction potential and amount of dissolved oxygen in aqueous solutions were measured during the reactions. The results show that the addition of Cr(III)₂(SO₄)₃ solution suppresses the transformation of GR2(SO₄²⁻) into iron oxyhydroxides and oxides in aqueous solution, while the addition of Na₂Cr(VI)O₄ solution promotes the transformation of GR2(SO₄²⁻) in which Cr(VI) is reduced to Cr(III); α-FeOOH particles were refined by the addition of the chromium ions.     

SECM Study of Effect of Chromium Content on the Localized Corrosion Behavior of Low-Alloy Steels in Chloride Environment

This paper investigates the effect of chromium (Cr) content (0, 1, 3 and 5% Cr) in epoxy-coated alloy steel against corrosion using in situ electrochemical techniques such as EIS and SECM in a 3% NaCl solution. The EIS results revealed that the epoxy-coated Cr steel exhibited higher impedance values than carbon steel, which is attributed to the greater resistance of Cr steel toward corrosion. Based on the cyclic voltammogram results, the tip potentials were set at ?0.7, 0.04 and 0.60 V for determining the concentration of dissolved oxygen at cathodic region, and oxidation of Cr²⁺ and Fe²⁺ at anodic region, respectively. The SECM measurements showed that, the tip current in the anodic region has decreased with increase in Cr content of the sample, which indicates that the oxidation of Fe²⁺ and Cr²⁺ decreases (corrosion is reduced) with the increase in Cr content of the steel. Besides, 5% Cr steel can maintain the highest corrosion resistance, and 1 and 3% Cr steels have higher corrosion resistance than the 0% Cr steel. This higher corrosion resistance of Cr steel samples could be due to the formation of Cr-rich hydro-oxide layers [Cr(OH)₃ as a corrosion product] on the surface of the samples. Thus, the epoxy-coated Cr alloy steel has greater corrosion resistance in a chloride-containing environment than the carbon steel. Hence, epoxy-coated Cr alloy steel can be successfully used as a construction material in structures.     

Electroplating of chromium coatings from Cr(III)-based electrolytes containing water soluble polymer

The effect of certain water-soluble synthetic polymers on the current efficiency, the kinetics of partial cathodic reactions, and certain properties of coatings at the chrome plating from sulfate-formate baths based on Cr(III) salts is studied. A polymeric additive of the nonionogenic type N1 (3 × 10⁶ g/mol) is proposed, which favors the deposition of chromium coatings that have good adhesion to the surface, do not crack, and have a thickness of several tens micrometers. It is found that the polymers have an accelerating effect on the aggregation of the particles of the chromium-hydroxide sol formed in the near-electrode layer, which, apparently, improves the deposit quality and enhances the current efficiency. It is shown that the resulting amorphous-crystalline chromium deposits are sufficiently hard and exhibit good protectiven ability at the anodic dissolution in aggressive media. Original Russian Text ? F.I. Danilov, V.S. Protsenko, T.E. Butyrina, E.A. Vasil’eva, A.S. Baskevich, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 6, pp. 603–612.     

Thermodynamic model for estimating the composition of passive films and Flade potential on Fe-Cr alloys in aqueous solutions

A thermodynamic model of a passive film is developed, in which the formation of the film on the surface of an Fe-Cr alloy in an aqueous solution is considered to be a result of the stable solid-phase chemical and adsorption equilibrium at the alloy-inner passive film layer interface. In the calculations, the Cr₂O₃ content in the passive film is determined by both the Gibbs energy change (ΔG < 0) in the chemical oxidation of the alloy components by the water oxygen and the change in the surface Gibbs energy (ΔG _S > 0) of the alloy. The ΔG _S change results in the negative adsorption of chromium atoms, which shifts the 3Fe + Cr₂O₃ ↔ 3FeO + 2Cr equilibrium toward the FeO formation in the passive film. Calculations showed that the enrichment of the passive film in chromium oxide should sharply increase in a chromium content range of 10–20% in the alloy, which agrees with the known data of X-ray photoelectron spectroscopy of the passive films. A formula is derived for estimating the Flade potential of Fe-Cr alloys, which relates the Flade potentials of individual Fe and Cr components to the FeO and Cr₂O₃ contents in the passive film.     

Economy in Metal Finishing

The effect of chromium and of nickel plating on the fatigue strength of steel was investigated. In the work on chromium a variety of steels of different compositions and ultimate tensile strengths was plated and, for unbaked deposits, a linear relationship between the percentage change in fatigue strength on chromium plating and the fatigue limit of the steel substrate was established. It is thus possible to predict the percentage change in fatigue strength for deposits of known internal stress from a knowledge of the fatigue limit, the ultimate tensile strength, or hardness of the steel, irrespective of its composition and a simple formula has been derived for this purpose.

The mode of failure of chromium plated components under alternating stress is discussed and there is evidence to suggest that for steels of low strengths (less than approximately 18 tons/in.² fatigue limit) failure initiates in the steel at the interface. With steels exceeding this strength the chromium fails first when the applied stress plus the internal stress of the deposit reaches the inherent fatigue limit of the chromium (20–22 tons/in.²). Propagation of the crack into the steel and consequent failure of the component does not appear to occur, however, until the stress concentration at the root of the crack approaches the fatigue limit of the substrate.

The effect of nickel deposits from two dull and three bright baths was studied and there is evidence to suggest that, apart from the effect of thickness, the broad conclusions arrived at in connection with chromium deposits apply for nickel.     

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. I. Influence of C,Mn, and Si

The oxidation behavior of Ni-Cr alloys (34 and 20 wt.% Cr) was investigated between 850 and 1200°C in oxygen for a maximum duration of about 70 hr. The oxide-growth mechanism is a diffusion process controlled by either outward diffusion of chromium in Cr₂O₃ (Ni-34Cr alloy) or by an increase in grain size (Ni-20Cr alloy). In the case of the Ni-34Cr alloy, low values of chromium diffusion were found for the growth of Cr₂O₃ by taking into account the general equation of Wagner. The influence of impurities (Si, C, Mn, Ni) diffusing from the underlying alloy is analyzed because of their doping effect in the outer oxide scales.     

Direct electrolytic preparation of chromium metal in CaCl2–NaCl eutectic salt

The electro-reduction of chromium oxide (Cr₂O₃) was investigated in an equimolar mixture of CaCl₂–NaCl molten salt at 800 °C for developing a more efficient process for chromium preparation. Cyclic voltammetry and potentiostatic electrolysis were used to study the electro-reduction of the Cr₂O₃-loaded metallic cavity electrode. In addition, a number of parameters affecting the rate and extent of Cr₂O₃ electrolysis were considered to better understand the electrolysis process. The results demonstrate that CaCl₂–NaCl molten salt is applicable for preparing Cr directly from Cr₂O₃ and the electrolysis parameters exert great influence on the cathode product. Under optimal experimental conditions, nodular Cr with an oxygen content of 0.5% (mass fraction) was obtained without any chromium carbides detected by XRD. Furthermore, the relatively high solubility of CaO and quite rapid crystal growth result in the formation of large platelet CaCr₂O₄, and the addition of NaCl to CaCl₂ results in several variations on the electrolysis process and the product morphology from pure CaCl₂ molten salt.     

A comparative study of the corrosion protective properties of chromium and chromium free passivation methods

Commercially available passivation methods for white-rust protection of hot-dip galvanized steel have been investigated. The passivations were either based on trivalent chromium or chromium free. A chromate based conversion coating was used for reference. The treated panels were tested with regard to white rust protection and paintability. The surface chemistry of the conversion coatings was monitored with scanning Auger electron spectroscopy and X-ray photoelectron spectroscopy. Coating thicknesses were measured using Auger electron sputter depth profiling.The passivations were applied with a thickness recommended by the supplier and thus showed large variation. The thickness of the chromium free passivation (Cr-free) is approximately 75 nm. The coating contains the active ions; H₃O⁺, Ti⁴⁺, Mn²⁺, Zn²⁺, PO₄³⁻. The passivation based on trivalent chromium (Cr-III) is approximately 30 nm thick and contains the active ions; H₃O⁺ Cr³⁺, PO₄³⁻, F. The chromate based passivation (Cr-VI) is approximately 5 nm thick and contains the active ions Cr⁶⁺/Cr³⁺, F⁻.The Cr-free and the Cr-III passivations showed similar white rust protection in the corrosion tests. The corrosion resistance was good although it did not fully reach the level of the Cr-VI passivation. The results from the tests of the painted panels showed that the powder paint worked well on all three passivations. The solvent born paint worked best on the passivation based on trivalent chromium. The water born paint showed poor resistance to blistering in the Cleveland humidity test for all three passivations. In this test the passivation with hexavalent chromium showed slightly better results than the chromate free passivations.     

Electrodeposition and tribological behavior of amorphous chromium-alumina composite coatings

The amorphous chromium-alumina (a-Cr-Al₂O₃) composite coatings with thickness of 50 μm were electrodeposited from Cr(III) electrolytes containing Al₂O₃ particles and Al³⁺. The effects of Al³⁺ on the dispersibility and Zeta potential of Al₂O₃ particles in electrotyles were investigated. It was found that the Al³⁺ promotes the Al₂O₃ particles to uniformly disperse in the Cr coatings. Tribological behavior of a-Cr-Al₂O₃ composite coatings was studied as compared with amorphous chromium (a-Cr) coatings electrodeposited from single Cr(III) electrolytes. The results demonstrate that the introduction of Al₂O₃ particles significantly improves the tribological performance of a-Cr coatings.     

Novel Alkaline Method for the Preparation of Low-Chromium Magnesia

The typical method for the preparation of chromium-magnesia is by energy-consuming grinding, but the pollution of Cr⁶⁺ ions is a problem. However, the chromium-magnesia refractory is still irreplaceable due to its outstanding performance. In this regard, low-chromium magnesia was prepared by the alkaline chromium precipitation method from cheap light-burned magnesia. The impurities in the light-burned magnesia was removed in order to avoid the formation of Cr⁶⁺. The chromium was precipitated directly on the light-burned magnesia surface without the grinding process. The low-chromium magnesia was obtained after the calcination. The reaction temperature, solid content, reaction time and stirring speed were examined and low-chromium magnesia with Cr³⁺ 0.4%, MgO 98.5% was obtained. The effect of temperature on the surface tomography of low-chromium magnesia was also discussed. The present novel alkaline method should be a promising way for the preparation of low-chromium magnesia.

     

Kinetics and Mechanism of Chromium Electroplating from Cr(III) Baths

Kinetics of step-wise discharge of Cr(III) to Cr(0) in electrolytes containing aminoacetic acid is studied. The limiting current of Cr(II) ion discharge to metal is much lower than the limiting diffusion current of Cr(III) reduction to Cr(II), which results in the accumulation of Cr(II) ions in the electrolyte even at the potentials of the limiting current of chromium electroplating. The apparent catalytic effect of OH^–ions during Cr(II) discharge is associated with the formation of electroactive hydroxocomplexes of bivalent chromium. The kinetic equation of the studied process is derived and analyzed. The probable nature of the limiting currents of chromium electroplating is discussed.